The instant invention relates to transparent iron oxide pigments. More specifically, the instant invention relates to a process for substantially improving the transparency of such pigments.
Iron oxide pigments are found in nature as mineral deposits. There are three types of iron oxide pigments which are found in nature as mineral deposits. These are limonite, hematite, and magnetite. Limonite is yellow and has a chemical formula of Fe.sub.2 O.sub.3.xH.sub.2 O where x is an integer. As can be seen from its formula, it is a hydrated iron(III)oxide. Hematite is red and has a chemical formula of Fe.sub.2 O.sub.3. It is an anhydrous iron(III)oxide. Magnetite is black and has a chemical formula of Fe.sub.3 O.sub.4. It is considered to be a spinel containing iron(II) and iron(III)ions.
In addition to the natural mineral deposits, iron oxide pigments can be obtained synthetically. The prior art preparation of iron oxide pigments is well known. Reference is made to various topics concerning iron oxide such as "Pigments (Inorganic)," pages 816-818, vol. 17, Encyclopedia of Chemical Technology, by Kirk-Othmer, 3rd Edition, published by John Wiley & Sons. Synthetic methods for preparing iron oxide pigments generally involve the alkali precipitation of iron(II) compounds from a soluble iron(II)salt and the oxidation of the precipitated iron(II) compound to an iron(III)oxide pigment slurry. The pigment produced by such methods corresponds to the yellow hydrated iron(III)oxide previously described. The yellow pigment is recovered from the slurry by filtration, washing, and drying. The alkali precipitation must occur under acidic conditions; otherwise, dark brown or black undesirable color shades will be produced. Iron oxide pigments of various yellow shades can be prepared by controlling the temperature and rate of oxidation. Red, orange and other colors can be prepared by calcining the yellow pigments at high temperatures.
Iron oxide pigments which have an average particle size less than 0.1 micron are considered to be transparent because they can transmit visible light. Stated another way, "transparent pigment" is pigment having a majority of the particles smaller than the wavelength of light. Iron oxide pigments which have an average particle size greater than 0.1 microns and which cannot transmit visible light are considered to be opaque.
Yellow and red transparent iron oxides are used in combination with aluminum flake pigments to produce automotive metallic top coats. The metallic effect is dependent on the degree of transparency of the pigment.
Iron oxide pigments produced by the alkali precipitation of iron compounds from a soluble iron salt contain salts as a result of the hydrolysis reaction which have to be subsequently washed after the pigment is filtered. Also because of the high surface tension of the water, the pigment undergoes agglomeration during drying which makes the pigment hard to disperse. Accordingly, it is the purpose of the instant invention to provide a process for producing extremely fine iron oxide pigments. It is an additional purpose of the instant invention to be able to produce extremely fine red iron oxide pigments during the normal processing, i.e., a one-step reaction whereby the necessity of calcining the yellow iron oxide pigment to get red is eliminated. It is also a purpose of the instant invention to be able to produce a metallic pigment for automobile top coats whereby fracturing of the aluminum flake particles is minimized and the aluminum flake is included during the normal process to produce the yellow or red iron oxide pigment.